1. Field of the Invention
The present invention relates to a telecentric imaging optical system which has a widely variable imaging magnification.
2. Description of the Related Art
The optical system disclosed in U.S. Pat. No. 4,867,545 can be cited as a conventional telecentric imaging optical system of which can be used at a wide range of imaging magnification. The telecentric imaging optical system revealed in the U.S. Patent requires that a first lens system and a third lens system each have a positive power at the object and the image formation sides and that the rear focal point of the first lens system approximately coincides with the front focal point of the third lens system. Another lens system having a positive power is disposed in the vicinity of these focal points. Hence, a distance between an object and the first lens system is shorter than the focal length of the first lens system and a distance between an image and the third lens system is shorter than the focal length of the third lens system. This in turn reduces a distance between the object and the image. In the optical system, it is easy to increase the numerical aperture.
However, the conventional imaging optical system has a drawback that the range of the variable power is relatively narrow. As can be seen from the U.S. Patent, the variable power ratio is 4 at most, obviously an insufficient ratio. The variable power ratio is a ratio of the maximum minification to the maximum magnification. When, for example, an imaging magnification ranges from -2 to -0.5, the imaging optical system has a variable power ratio of 4.
A further problem with the conventional imaging optical system is as follows. Since the conventional imaging optical system is comprised of the three positive power lens systems, it is theoretically concluded that the Petzval's summation tends to be large in the optical system. If the Petzval's summation is large, there arises greater chances that the image plane is not flat. Therefore, it is difficult to enlarge the size of an image which will be eventually formed.